It is known to form containers made of thermoplastic material by the stretch blow molding of preforms that are heated in advance to a temperature that is high enough to soften the walls thereof.
For this purpose, a forming device is used that comprises a mold that has a cavity in accordance with the impression of the container that is to be obtained. The previously heated preform is accommodated in the cavity. Then, its walls are subjected to a so-called “biaxial” stretching to assume the impression of the mold. To do this, the preform is stretched axially by means of a stretching rod to bring about the axial expansion of the preform. Simultaneously to this stretching operation, a pressurized fluid is injected into the preform in such a way as to bring about the radial expansion of the wall.
Such a forming method is well known. So that the final container has a perfectly molded bottom, it is preferable that the stretching rod brings the bottom of the preform into contact with the mold bottom. It is preferable that the wall that forms the bottom of the preform is slightly pinched between the free end of the stretching rod and the mold bottom.
For the large-scale production of containers, it is known to use a blow-molding machine, in particular a rotary blow-molding machine, which is equipped with a number of forming devices of this type.
Furthermore, such a forming device is very expensive. To make it possible for container manufacturers to maximize their returns from this equipment, the same molding device is designed for making it possible to produce containers of different formats by replacing certain so-called “personalizable” elements, such as the molding elements.
To reduce the manufacturing cost of such a machine, certain components are used independently of the format of the container that is to be produced. This is in particular the case of the drive element of the stretching rod. Thus, it is provided to be able to adjust the travel of the stretching rod based on multiple parameters, among which there will be cited by way of non-limiting examples the format of the container that is to be produced and the thickness of the wall of the preform.
To date, the adjustment of the travel of the stretching rod is carried out by an operator who, in preparation for the production of a series of containers of a determined format, programs, for example, a control unit of the stretching rod to make it slide on a determined course.
However, it may happen that the operator makes an error or else that the thickness of the bottom wall of the preforms and/or the dimensions of the molds are not in compliance with the specifications.
In this case, it may happen that the stretching rod abuts against the mold bottom and/or that it forces the preform wall too heavily against the mold bottom. To prevent the motor for controlling the sliding of the stretching rod from being damaged, in particular because of overheating, and to prevent the containers that are obtained from having anomalies, it is known to monitor the axial force exerted by the stretching rod on the preform, in particular close to its stretching end position.
When the axial force that is detected is too high, it is considered that the travel of the stretching rod is too long. It is generally expected that an abnormally high axial force occurs several times before a problem is indicated, in particular to avoid temporary anomalies.
When the problem is indicated, the operator stops the intake of preforms in the blow-molding machine, and then the blow-molding machine is halted in such a way as to correct manually the length of the travel of the stretching rod.
During this operation, the preforms that are already present in the blow-molding machine are scrapped because they were not able to have been formed correctly. This operation therefore leads to the raw material being wasted.
In addition, such an operation involves the total halting of the production line for the period that is necessary to correct the travel. This operation therefore leads to a loss of time.